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Chemistry LibreTexts

3: Atoms and the Periodic Table

  • Page ID
    • 3.1: Development of the Modern Periodic Table
      The periodic table is an arrangement of the elements in order of increasing atomic number. The periodic table is one of the cornerstones of chemistry because it organizes all the known elements on the basis of their chemical properties. Elements that exhibit similar chemistry appear in vertical columns called groups (numbered 1–18 from left to right); the seven horizontal rows are called periods.
    • 3.2: Looking for Patterns: The Periodic Table
      Certain elemental properties become apparent in a survey of the periodic table as a whole. Every element can be classified as either a metal, a nonmetal, or a metalloid (or semi metal). A metal is a substance that is shiny, typically (but not always) silvery in color, and an excellent conductor of electricity and heat. Metals are also malleable (they can be beaten into thin sheets) and ductile (they can be drawn into thin wires).
    • 3.3: Indivisible - The Atomic Theory
      You learned earlier how all matter in the universe is made out of tiny building blocks called atoms. All modern scientists accept the concept of the atom, but when the concept of the atom was first proposed about 2,500 years ago, ancient philosophers laughed at the idea. It has always been difficult to convince people of the existence of things that are too small to see. We will spend some time considering the evidence (observations) that convince scientists of the existence of atoms.
    • 3.4: The Properties of Protons, Neutrons, and Electrons
      Electrons are extremely small. The mass of an electron is only about 1/2000 the mass of a proton or neutron, so electrons contribute virtually nothing to the total mass of an atom. Electrons have an electric charge of −1, which is equal but opposite to the charge of a proton, which is +1. All atoms have the same number of electrons as protons, so the positive and negative charges "cancel out", making atoms electrically neutral.
    • 3.5: Activity: Build an Atom
      Build an atom out of protons, neutrons, and electrons, and see how the element, charge, and mass change. Then play a game to test your ideas!
    • 3.6: Atomic Number
    • 3.7: Mass Number
    • 3.8: Isotopes
    • 3.9: Atomic Models of the Twentieth Century
      In 1913, the Danish physicist Niels Bohr proposed a model of the electron cloud of an atom in which electrons orbit the nucleus and were able to produce atomic spectra. Understanding Bohr's model requires some knowledge of electromagnetic radiation
    • 3.10: Bohr's Atomic Model
    • 3.11: Understanding Atomic Spectra
      The ground state of an atom is the lowest energy state of the atom. When those atoms are given energy, the electrons absorb the energy and move to a higher energy level. An excited state of an atom is a state where its potential energy is higher than the ground state. When it returns back to the ground state, it releases the energy that it had previously gained often in the form of electromagnetic radiation (although it can be released via heat).
    • 3.12: Predicting Ion Charges
      Most atoms do not have eight electrons in their valence electron shell. Some atoms have only a few electrons in their outer shell, while some atoms lack only one or two electrons to have an octet. In cases where an atom has three or fewer valence electrons, the atom may lose those valence electrons quite easily until what remains is a lower shell that contains an octet.

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